Method for producing phosphor screen

ABSTRACT

A phosphor screen for color picture tube is usually produced by the first step which comprises suspending a blue emission phosphor in an aqueous solution of photo-setting photosensitive material comprising polyvinyl alcohol and ammonium dichromate, applying thus obtained suspension to inner surface of face panel to form a photosensitive film, exposing said film through apertures of shadow mask and developing the exposed film to form dotty phosphor film and the second and the third steps of forming the dotty phosphor films of green and red emissions in this order by substantially the same procedure as that of the first step. According to the present invention, the dotty phosphor films formed at said first and second steps are treated with chromic acid solution to harden said films, whereby color mixing, namely, so-called color fogging caused by a phosphor of other color emission used at the subsequent step is reduced and color purity of emission from phosphor screen face is improved.

United States Patent 91 Kobayashi et a1.

[ Aug. 21, 1973 METHOD FOR PRODUCING PIIOSPIIOR SCREEN [75] Inventors: Yoshichika Kobayashi, Ibaragi;

Kouzi Yakeno, Katano, both of Japan [73] Assignee: Matsushita Electronics Corporation,

Osaka, Japan [22] Filed: Dec. 13, 1971 {21] Appl. No.: 207,180

[30] Foreign Application Priority Data Dec. 18, 1970 Japan 45/116099 [52] U.S. Cl. 96/36.1, 96/111 [51] Int. Cl G03c 5/00 [58] Field of Search 96/36.l, 111; 8/9427 [56] References Cited UNITED STATES PATENTS 3,428,454 2/1969 Angelucci 96/36.l 1,763,533 6/1930 Miller et al. 96/111 2,766,098 10/1956 Perrin 8/9427 [57] ABSTRACT A phosphor screen for color picture tube is usually produced by the first step which comprises suspending a blue emission phosphor in an aqueous solution of photo-setting photosensitive material comprising polyvinyl alcohol and ammonium dichromate, applying thus obtained suspension to inner surface of face panel to form a photosensitive film, exposing said film through apertures of shadow mask and developing the exposed film to form dotty phosphor film and the sec- 0nd and the third steps of forming the dotty phosphor films of green and red emissions in this order by substantially the same procedure as that of the first step. According to the present invention, the dotty phosphor films formed at said first and second steps are treated with chromic acid solution to harden said films, whereby color mixing, namely, so-called color fogging caused by a phosphor of other color emission used at the subsequent step is reduced and color purity of emission from phosphor screen face is improved.

3 Claims, 3 Drawing Figures METHOD FOR PRODUCING PHOSPHOR SCREEN The present invention relates to a method for producing a phosphor screen which comprises forming multi-color emission phosphor dots on the inner surface of a face panel by photo-mechanical process utilizing photo-setting type photosensitive film.

Generally, phosphor screen in color picture tube is produced by the steps which comprise mixing polyvinyl alcohol and a dichromate such as ammonium dichromate as a photosensitizer in a predetermined ratio, dispersing a phosphor in said mixed liquid, uniformly applying the resultant suspension to the inner surface of the face panel, drying the applied suspension to form a photosensitive film, exposing said film in dotty form through apertures of a shadow mask and then developing the exposed film. According to such method, regular arrangement of dots of the tri-color emission phosphors is attained by respectively transferring the position of light source for exposure to the deflection center of electron beam for excitation of each phosphor and repeating the above mentioned each step using green, blue and red emission phosphors.

Among dots of multi-color emission phosphors obtained by said method, phosphor of different color emission necessarily adheres to the phosphor dots obtained at the first and the second steps except the phosphor dots obtained at the last step and it is difficult to make the adhesion amount of the different color phosphor zero. That is, according to the usual method for forming phosphor dots, it is impossible to completely remove, at the second and the third developing steps, the phosphor of different color emission which adheres to the surface of phosphor dots formed at the preceding step. if development for a long period is carried out to completely remove the adhered phosphor, even the phosphor dots adhering inner surface of face panel at the preceding step are also removed, namely, falling of dots is caused.

The object of the present invention is to provide an effective means for preventing color-mixing of each phosphor to obtain color picture tube having excellent images.

FIGS. 1, 2 and 3 are graphs which show relations between Cro concentration and colorfogging rate, between CrO, concentration and luminance and between Cr0, concentration and practicality, respectively.

The method of the present invention will be more detailedly explained by means of the following Example.

0.01 025 percent By weight of polyvinyl alcohol of ammonium dichromate was added to an aqueous solution containing 1-7 percent of polyvinyl alcohol to form a photo-setting type photosensitive film forming liquid. The first phosphor, e.g., a green emission phosphor was suspended in said film forming liquid. Thus obtained suspension was uniformly applied to the surface of face panel by the known flow coating method. Thus applied layer was dried to form photosensitive film and then this film was exposed to dotty light source placed at the position corresponding to deflection center of electron beam through apertures of shadow mask fixed at the face panel and thereafter unexposed portions of the film were flowed away with warm water and said exposed portions were dried to obtain dotty film containing the phosphors.

Thereafter, said dotty film was subjected to dipping treatment with chromic acid solution of a predetermined concentration for several ten seconds and then thus treated film while in wet state was again washed with warm water.

Hardening of said dotty film was extremely accelerated by said treatment with chromic acid solution and color mixing rate, namely, degree of color fogging of each phosphor dots on a phosphor screen is lowered as compared with those of phosphor dots formed without said treatment. According to experiments, the degree of hardening of the film was conspicuous at a pH of less than 2 of the chromic acid solution. FIG. 1 shows relation between concentration of chromic acid (Cr0,) in said treating solution and color fogging as a relative value taking as the amount of blue phosphor incorporated into green phosphor dots when no treatment with chromic acid solution was given.

According to the results of said experiments, with increase of the concentration of Crtl up to about 10 percent, hardening of the film proceeds and color fogging tends to decrease. However, there is another phenomenon that luminance decreases when chromiun adheres to phosphor. FIG. 2 shows the relation between Cr0 concentration in the treating solution and luminance measured after baking at 500C for one hour on a green emission phosphor, namely, copper aluminum activated zinc sulfide cadmium sulfide phosphor [(Zn, Cd)S Cu, Al] used in the present invention. The result indicates that Cr0 concentration exceeding 10 percent is not preferred. FIG. 3 shows a practical curve of Cr0 concentration in the treating solution obtained considering decrease of color fogging caused by film hardening treatment and allowable lower limit of luminance of the phosphor. That is, practical Cr0,-, concentration is up to 10 percent, preferably 0.5-10 percent, within which film hardening and allowable decrease of luminance are balanced. The peak of practical value is seen at a CrO concentration of about 5 percent. The treatment with chromic acid solution is applied to the step of forming dots of the second phosphor, e.g., blue emission phosphor, whereby hardening of dotty film 'containing blue emission phosphor can be accelerated. Furthermore, since the phosphor screen of the usual color picture tube have tricolor phosphors, there is no necessity to apply said treatment with chromic acid solution to the step of forming phosphor dots of the third phosphor, e.g., red emission phosphor. The treatment with chromic acid solution has the same film hardening effect even if the order of forming the dots of the first, the second and the third phosphors is changed. However, preferably, such phosphor as exhibiting greater decrease of luminance duento adhesion of chromium should be employed later. Especially, when dots of red emission phosphor of a rare earth oxide are lastly formed, decrease of luminance due to adhesion of chromium is prevented to obtain bright phosphor surface.

Even when a suspension of other phosphor is applied to a hardened dotty film and dried, the unexposed photosensitive film can be completely removed by washing with warm water.

As explained before, the method of the present invention is characterized by the steps of applying a suspension of a given phosphor and a photo-setting type photosensitive material comprising polyvinyl alcohol and a dichromate to an inner surface of a face panel to form a photosensitive film, exposing and developing said film in dotty form to obtain a dotty film containing the given phosphor and subjecting the resultant dotty film to dipping treatment with chromic acid solution to harden the film.

The method of the present invention may also be employed when a phosphor screen is formed by the abovementioned steps of dots formation on a face panel having holes at the position on one surface at which dots of tricolor emission phosphors are to be formed and light absorbing film such as graphite on the other surface.

Furthermore, the present invention may be applied to production of color picture tube having a phosphor screen of stripe form, when polyvinyl alcoholdichromate system is used as the photosensitive film.

What is claimed is:

l. A method for producing a phosphor screen which comprises the steps of applying a suspension of a photo-setting type photosensitive material comprising polyvinyl alcohol and a dichromate and a phosphor to a surface of a screen forming plate to form a photosensitive film, exposing to actinic radiation and developing said film in a repetition pattern such as dotted form or stripe form to form phosphor film of the certain repetition pattern, and contacting said phosphor film with an aqueous solution of chromic acid having a pH of not more than 2.0 to harden the film.

2. A method according to claim 1, wherein said chromic acid solution is an aqueous solution containing an optional amount of up to 10 percent by weight of chromic acid.

3. A method for producing a phosphor screen of color picture tube which comprises the steps of applying a suspension of a phosphor and a photo-setting type photosensitive material comprising a polyvinyl alcohol and a dichromate to an inner surface of face panel of a cathode-ray tube to form a photosensitive film, exposing to actinic radiation and developing said film in dotty form to obtain dotty phosphor film and then allowing said film to contact with a chromic acid aqueous solution containing 0.5-10 percent by weight of chromic and having a pH of not more than 2.0 acid to harden the film. 

2. A method according to claim 1, wherein said chromic acid solution is an aqueous solution containing an optional amount of up to 10 percent by weight of chromic acid.
 3. A method for producing a phosphor screen of color picture tube which comprises the steps of applying a suspension of a phosphor and a photo-setting type photosensitive material comprising a polyvinyl alcohol and a dichromate to an inner surface of face panel of a cathode-ray tube to form a photosensitive film, exposing to actinic radiation and developing said film in dotty form to obtain dotty phosphor film and then allowing said film to contact with a chromic acid aqueous solution containing 0.5- 10 percent by weight of chromic and having a pH of not more than 2.0 acid to harden the film. 